Realistic Rusty Chain
Seams are visible around the link at the lines corresponding to the UV seams :



To correct that, you can paint (texture paint) this part using the clone brush, preferably using a strength of 1 (if not you will have more blurry painting).

To clone paint, Ctrl+LMB at the position you want to clone, then paint along the seam :



Note that will change your texture. So make a copy of it before you do that.

For adding more rusty parts, you can use the same principle, both in order to have more rust at the places you want and also to correct the seams :



The same part, before and after (no need to be an artist to do that !):



Edit : to have more rust in a more easy way (but that will not correct the seam problem) you can move the UV map parts to the center. In this particular case, that will not have any impact for the other texture :